The invention pertains to methods and apparatus for modifying the refractive properties of the eye. More particularly, the invention pertains to methods and apparatus for correcting refractive errors by modifying the cornea of the eye.
In general terms, the human eye functions by sensing light rays. Such light rays tend to be focused as they pass through the cornea, the aqueous humor, the lens and the vitreous humor. Ideally, the focal point of light, after passing through these components, will be at the retina. Emmetropia, or the lack of refractive error, is thus characterized by the focal point of the light entering the eye from an infinite distance and falling on the retina.
FIG. 1 illustrates the condition of emmetropia. As shown in FIG. 1, light enters the eye through the cornea 61 and passes through the cornea, the aqueous humor 2, the crystalline lens 3 and the vitreous humor 6. The light is focused by the refractive power of the cornea 61, the aqueous humor 2, the crystalline lens 3 and the vitreous humor 6 to a focal point Pl which, in the case of emmetropia as shown in FIG. 1, is at the retina 4. The globe of the eye is generally indicated at numeral 5 in FIG. 1.
Billions of human beings suffer impaired vision due to refractive errors of the eye characterized by the focal point of light failing to be at the retina, but rather falling short of or behind the retina. Common refractive errors of the eye fall into three main categories: myopia, hyperopia, and astigmatism. Myopia (FIG. 2), also known as nearsightedness, results when the focal point P2 of the eye is located anterior to the retina 4. Hyperopia (FIG. 3), also known as farsightedness, results when the focal point of the eye is located posterior to the retina 4. Astigmatism results when the eye has different refractive errors at different meridians. Thus, astigmatism may be present as a combination of any two of emmetropia, myopia and hyperopia in the same eye. For example, in an astigmatic eye, light entering the eye in a horizontal meridian may be focused anterior to the retina, while light entering the eye in a vertical meridian may be focused posterior to the retina.
As is well known, the cornea provides approximately two-thirds (2/3) of the refractive power of the eye. This is primarily due to the optically powerful air/cornea interface created by the large disparity of refractive indices between the air (1.00) and the cornea (approximately 1.37). The aqueous/lens interface causes further refraction within the eye.
Because the cornea is such an important factor in refraction of the eye, a wide variety of method and apparatuses have been applied in the past to alter the cornea in an effort to eliminate refractive errors. For example, contact lenses, which are also commonly used as refractive entities in themselves, have been intentionally malfitted to temporarily alter the corneal curvature. The latter technique is known as "orthokeratology" and generally results in only a temporary change in the corneal curvature. Orthokeratology has a further deficiency in that it is known to induce potentially serious corneal inflammation and scarring.
Several other techniques are known for altering the cornea in various ways to compensate for refractive errors of the eye. For example, radial keratotomy involves the making of radially orientated slit-like incisions in the cornea, in various patterns, to attempt to correct myopia and/or astigmatism. At present, however, the results of radial keratotomy are unpredictable and are often not reproducible in the same patient. Additionally, it is as yet unclear how long the results of radial keratotomy last. Further, there have been reports of corneal degenerations, infections and distortions after radial keratotomy, such conditions obviously having the potential for serious visual loss.
It is also known to use lasers for altering the condition of the cornea. U.S. Pat. No. 4,461,294 illustrates the use of the thermal effect of a laser to induce corneal-recurving scars by imbedding, under pressure, light absorbing colored bodies in the cornea in a radial pattern. The colored bodies in the cornea are exposed to a thermal laser through a matched, slitted diaphram In the technique disclosed by the U.S. Pat. No. 4,461,294, corneal tissue is burned for the purpose of creating scar tissue
Another technique for modifying the cornea, known as lamellar keratoplasty involves the taking of a slice of a patient's cornea, or a donor's cornea, freezing the portion and lathing it in a hard-frozen state to a new curvature prior to suturing onto the eye of the patient. Particular methods employing this technique include keratomileusis, keratophakia, and epikeratophakia, each of which requires cutting and suturing of the patient's cornea.
Yet another cornea modification technique is disclosed in U.S. Pat. No. 4,665,913 which discusses a device for exposing the cornea to an excimer laser in perpendicular fashion to reshape the cornea. The U.S. Pat. No. 4,665,913 patent discloses a laser which is directed at the eye substantially along the visual axis of the eye. Removal of tissue is effected by exposing the cornea head-on to varying flux densities and exposure time in either rectilinear or spiralling fashion. This head-on exposure to the radiation of the laser would presumably expose the eyes delicate internal structures, such as the iris, the lens and the retina, to potentially damaging levels of radiation. Additionally, in such a device, if the output of the laser were inadvertently increased, deeper levels of tissue penetration could result in accidental perforation of the cornea or irregular corneal refracting surfaces.
In U.S. Pat. No. 4,724,522 (Feb. 9, 1988), which is hereby incorporated-by-reference, I describe methods and apparatus for improving corneal refractive properties, these methods and apparatus providing tangential striking of the cornea with a laser beam. FIG. 6 herein is a partial elevational view of one embodiment of a laser delivery device according to U.S. Pat. No. 4,724,522. Shown in FIG. 6, positioned about a human eye 5 including cornea 61, are a laser beam 79, and a variable, optical element M.sub.3. FIG. 6 demonstrates six representative positions of optical element M.sub.3, namely M.sub.3A-F. In the practice of the embodiment of FIG. 6, the positions M.sub.3A-F would be calculated to result in the ablating beam touching the cornea tangentially at different positions to effect a new anterior corneal curvature.
Suggested surgical procedures for modifying corneal refractive properties in treating the conditions of myopia and hyperopia with an apparatus according to my earlier invention of U.S. Pat. No. 4,724,522 are described by way of reference to FIGS. 4A, 4B and 5 herein. FIGS. 4A, 4B and 5 illustrate schematic cross-sectional views of the eyes of a myopic correction (FIG. 4A with a smaller optical zone and FIG. 4B with a larger optical zone) and a hyperopic correction (FIG. 5). In FIGS. 4A, 4B and 5, RC.sub.I depicts the initial or pre-operative corneal radius of curvature, RCF depicts the final or post-operative corneal radius of curvature, T.sub.I depicts the initial or pre-operative corneal thickness and T.sub.F depicts the final or post-operative corneal thickness.
For treatment of the myopic patient (FIG. 4A), corneal tissue 40 may be removed from the apex 41 outward to the periphery 42, or from the periphery inward to the corneal apex, altering the cornea from its initial corneal thickness T.sub.I and initial corneal radius of curvature RC.sub.I to a lessened final corneal thickness T.sub.F and an increased final corneal radius of curvature RC.sub.F. Treating myopia (FIG. 4A) in such a procedure should thus result in a flatter cornea. FIG. 4B demonstrates the same RC.sub.F but with a larger optical zone diameter 43'.
In correcting or treating hyperopia (FIG. 5), the resultant corneal radius of the curvature should be smaller, resulting in a steeper cornea. Thus, in treating the hyperopic patient, the corneal radius of curvature and corneal thickness are both reduced as corneal tissue 50 is removed primarily from the mid-pheriphery 52 as illustrated in FIG. 5.
It is an object of this invention to provide an alternative method and apparatus for accurately shaping the cornea to compensate for refractive errors of the eye.
It is a further object of the invention to provide a method and apparatus for reshaping the cornea of the eye in vivo or in vitro without the need for removing and then suturing the corneal tissue.
It is a further object of the invention to provide a method and apparatus for shaping the cornea of the eye in which there is no necessity for freezing the corneal tissue prior to shaping but which may be used in conjunction with freezing techniques.
It is a still further object of the invention to provide a method and apparatus for shaping the cornea which reduces or eliminates the risk of accidental damage to the cornea and the other components of the eye, such as the iris, the lens and the retina.
It is a still further object of the invention to provide a method and apparatus which meets the foregoing objectives and which is safe, predictable, and reproducible.